A methanol carbonylation process is known as a process for industrially producing acetic acid. According to this process, for example, methanol is allowed to react with carbon monoxide in the presence of a catalyst in a reactor to give acetic acid in a reaction mixture, the resulting reaction mixture is separated, using an evaporator, into a vapor phase including acetic acid and light ends, and a residual liquid phase (residue phase) including acetic acid and the catalyst, the vapor phase is subjected to distillation in a distillation column (light ends column) to be separated into an overhead stream including light ends, and an acetic acid stream, and the acetic acid stream is further purified to give an acetic acid product. In the process, acetaldehyde is by-produced during the reaction, and the acetaldehyde causes the acetic acid product to have lower quality.
The acetaldehyde is converted into crotonaldehyde by aldol condensation and causes the acetic acid product to yield a worse potassium permanganate test result (permanganate time). In addition, crotonaldehyde is converted into 2-ethylcrotonaldehyde by aldol condensation with acetaldehyde; and 2-ethylcrotonaldehyde also causes the acetic acid product to yield a worse potassium permanganate test result. However, as compared with 2-ethylcrotonaldehyde, crotonaldehyde more worsens the potassium permanganate test result per mass unit, and when contained in the acetic acid product, more significantly worsens the quality of the product.
To decrease acetaldehyde, crotonaldehyde, and/or 2-ethylcrotonaldehyde, roughly classified two techniques, namely, techniques (i) and (ii) as follows, have been conventionally industrially employed (Patent Literature (PTL) 1 and PTL 2). In the technique (i), the formation of crotonaldehyde in the reaction system is restrained by liquid-liquid separating a condensate into an aqueous phase and an organic phase using a decanter, where the condensate is derived from an overhead stream from a light ends column, subjecting, of the two phases, the aqueous phase to distillation in an acetaldehyde-removing column, extracting a condensate with water to separate and remove acetaldehyde from the condensate, where the condensate includes acetaldehyde and methyl iodide and is derived from an overhead stream from the acetaldehyde-removing column, and thereby decreasing acetaldehyde in methyl iodide to be recycled to the reaction system. In the technique (ii), crotonaldehyde is directly decomposed oxidatively with ozone, where the crotonaldehyde is contained in a crude acetic acid which is obtained in the middle of a purification step. However, facilities for the acetaldehyde separation and removal, and facilities for the ozone treatment are both expensive. Conventional acetic acid production processes entirely depend on these techniques so as to give an acetic acid product that yields a better potassium permanganate test result, and this leads to increase in installation cost.
The methanol-acetic acid process is known to give alkanes as impurities. Such alkanes as impurities contain 3 or more carbon atoms and have higher boiling points as compared with methyl iodide and methyl acetate. The alkanes are mainly saturated or unsaturated hydrocarbons, but may contain an oxygen atom and/or an iodine atom in the molecule. Japanese Unexamined Patent Application Publication (JP-A) No. H04-295445 discloses a technique for the removal of the alkanes. In the technique, of overhead condensates from a light ends column, an organic phase is subjected to distillation in a distillation column (alkane-removing column), to give an overhead product including methyl iodide, methyl acetate, and carbonyl impurities, and bottoms including alkanes, water, and acetic acid; the overhead product is recycled to the reactor or fed to an acetaldehyde-removing column, the bottoms are combined with and extracted with water to give an aqueous phase including acetic acid, and an organic phase including alkanes; the aqueous phase is recycled to the reactor, and the organic phase including the alkanes is discarded as a waste. This literature, however, neither discloses nor indicates how to give an acetic acid product that yields a better potassium permanganate test result.